Arrays of relatively small structures, sometimes referred to as microneedles or micro-pins, have been disclosed for use in connection with the delivery and/or removal of therapeutic agents and other substances through the skin and other surfaces.
The vast majority of known microneedle arrays include structures having a capillary or passageway formed through the needle. Because the needles are themselves small, the passageways formed in the needles must be limited in size. As a result, the passageways can be difficult to manufacture because of their small size and the need for accurate location of the passageways within the needles.
Another potential problem of passageways small enough to fit within the microneedles is that the passageways may become easily obstructed or clogged during use.
As a result, a need exists for microneedle arrays that include fluid passageways that are easier to manufacture and that are resistant to obstruction or clogging during use.
Among the uses for microneedle arrays, penetration of skin is one commonly-discussed application. Skin is a three-layer protective barrier between the body and the outside world. At approximately 200 um thick, the epidermis is the thinnest, outermost layer of the skin and it contains many of the components that give skin it barrier-like characteristics. The outermost layer of the epidermis, the stratum corneum, is a thin layer (10-50 um) of flattened, dead cells, water, and lipids that helps the body retain water and prohibits the entrance of microorganisms and toxic chemicals. The stratum corneum, sometimes called the “horny layer” is both tough and flexible, with a significant degree of elasticity. These characteristics make the stratum corneum an effective barrier, resistant to penetration. There is significant variability in the thickness and elasticity of the stratum corneum associated with age and location on the body. For example, the stratum corneum of the feet is over ten times thicker than that found on the forearm of a typical human.
Beneath the epidermis is the dermis which houses blood vessels and nerve endings, hair shafts and sweat glands. Thousands of small capillaries (loop capillaries) feed the upper levels of the dermis, beneath the epidermis. These capillaries extend just above most of the nerve endings that also reside in the dermis. The deepest layer of skin, the hypodermis, insulates the body from extreme temperatures and provides a mechanical cushion from outside assaults. The hypodermis contains larger blood vessels and arteries and more nerves.
Delivery of substances into the skin or removal of fluids through the skin may be facilitated by the use of microneedle arrays. One problem associated with penetration of skin by microneedle arrays is, however, the viscoelastic properties of skin. When subjected to static or slow-moving loads, skin elongates before rupture.
As a result, many situations requiring the extraction of fluids, e.g., blood-glucose monitoring, required the use of sharp instruments such as lancets that pierce the skin. Such devices are, however, relatively painful to use and may pose a risk of inadvertent piercing of skin. Further, the pierced site may experience unnecessary bleeding.